This specification refers to and describes content of U.S. Pat. Nos. 4,346,314, 5,144,630 and 5,592,325 and international patent application PCT/AU98/00554 (WO 99/04317). However, neither the disclosures in those US patents and international patent application nor the descriptions contained herein of those US patents and international patent application are to be taken as forming part of the common general knowledge solely by virtue of the inclusion herein of the reference to and description of content that those U.S. patents and international patent application. Furthermore, this specification describes aspects of prior art lasers. However, neither such aspects of prior art lasers nor the description contained herein of such aspects of prior art lasers is to be taken as forming part of the common general knowledge solely by virtue of the inclusion herein of reference to and description of such aspects of prior art lasers.
Excimer gas lasers with an operating wavelength of 193 nm in the ultraviolet (UV) region of the electromagnetic spectrum have been utilised in many of the above applications. The short UV wavelength of these lasers processes material through photoablation. The material being processed is vaporized by the laser but little thermal damage is caused to adjacent areas. This has led to the widespread use of excimer lasers in the medical field. However, excimer lasers do have disadvantages. These disadvantages include poor reliability, high operating costs, and the need to use an extremely toxic gas. The gas also has a limited lifetime in the laser cavity and so must be replaced frequently. This adds the difficulties associated with handling and shipping a dangerous gas to the excimer laser disadvantages.
On the other hand, solid-state lasers are smaller, more reliable and easier to use than gas excimer lasers. These lasers utilize glass or crystal matrices, such as yttrium aluminium garnet (YAG), yttrium lithium fluoride (YLF) or potassium gadolinium tungstate (KGW) that are doped with rare-earth elements such as neodymium (Nd), erbium (Er) or holmium (Ho). Solid-state lasers are identified by the element and glass or crystal used. For example, a laser using a YAG crystal doped with neodymium is denoted Nd:YAG. This material is referred to as the laser medium. Excitation of the laser medium, usually by either flash lamp or diode lasers, results in high-energy laser emissions. These high-energy laser emissions have a variety of wavelengths. The rare-earth element in the laser medium determines the wavelengths that are produced. However, none of these solid state lasers produce laser emissions that are in the UV wavelength range of the laser emissions produced by excimer lasers. Some of the more common solid state lasers and the wavelengths of their laser emissions are Nd:KGW at 1.067 microns, Nd:YAG at 1.064 microns, Nd:YLF at 1.053 microns, Ho:YAG at 2.1 microns and Er:YAG at 2.94 microns. These are all in the infra-red portion of the electromagnetic spectrum, i.e. they have a (relatively) much longer wavelength than that of gas excimer lasers.
Whilst solid state lasers produce beams having longer wavelengths than those of gas excimer laser, they have been successfully applied to different medical and industrial processes. Even so, the longer infra-red wavelengths produced by solid state lasers makes them unsuitable for most of the applications using excimer lasers. Furthermore, they may produce undesirable effects when applied to some materials, such as corneal tissue.
It is possible to use non-linear optical (NLO) crystals to convert the infra-red wavelengths produced by solid state lasers, to shorter visible and ultraviolet wavelengths. U.S. Pat. No. 5,144,630 describes the use of non-linear optical crystals for frequency conversion of high intensity laser emission. This property of NLO crystals means that passage of a laser beam through such a crystal can result in the wavelength of the beam being altered. This property enables the laser beam produced by an infra-red laser, such as Nd:YAG at 1064 nm, to be converted to a shorter wavelength of 532 nm. This process is known as harmonic generation (and is described in U.S. Pat. No. 5,592,325 and U.S. Pat. No. 4,346,314). Converting an original infra-red laser beam, at 1064 nm, to a beam with a wavelength at 532 nm is known as second harmonic generator (SHG). The ability to generate higher harmonics, such as the fourth and fifth harmonic wavelengths of a Nd:YAG laser, at 266 nm and 213 nm, respectively, means that the solid state laser becomes suitable for further applications.
There is a wide range of non-linear optical crystals that can be used for harmonic generation to shorter wavelengths. Examples are crystals of the borate family, and include beta barium borate (β-BaB2O4 or BBO), lithium borate (LBO), caesium lithium borate (CLBO), MBeBo3F2 and CsB3O5. Other examples of NLO crystals for harmonic generation include Potassium Titanyl Phosphate (KTP or KTiOPO4) and potassium Dideuterium Phosphate (KD*P or KD2PO4) (as described in U.S. Pat. No. 5,144,630 and U.S. Pat. No. 5,592,325).
For the harmonic generation process to work properly, the laser beam must pass through the non-linear crystal at exactly the right angle relative to the crystal structure. A very small error in the angle that the laser beam passes through the crystal can cause the conversion efficiency to drop significantly, possibly even to zero. Fundamental problems exist with non-linear optical crystals. Firstly, the exact required angle through the crystal usually depends on the temperature of the crystal and temperature gradients within the crystal. Secondly, the crystal usually absorbs a little of either or both the incident longer wavelength and the newly generated harmonic shorter wavelength. This absorbed laser energy heats the crystal, changing its temperature and creating temperature gradients within the crystal. Thus, the required angle through the crystal for efficient harmonic generation when the crystal is cold, i.e. at the time the laser has just been switched on, is different from the required angle when the laser has been running for a while and its heating of the crystal has reached a steady state. When a laser is first switched on and the laser beam passes through the crystal at the angle required for warm steady state efficient harmonic generation, it is not unusual for there to be no harmonic generation at all. In such an instance, the harmonic wavelength cannot contribute to heating of the crystal, and therefore the temperature state of the crystal that produces any harmonic generation is never reached. Even when the differences in angles between the cold starting condition and the warm steady state condition are not sufficient enough to create the problem described above, the changes in optimum angle do create long warm-up times and potentially large swings in the energy of the generated harmonic wavelength. To reach the fourth or fifth harmonic, for example 266 nm or 213 nm for Nd:YAG, the conversion process usually requires two or three crystal stages respectively. The instabilities of energy are thus multiplied for these shorter wavelengths. Therefore these solid state UV wavelength laser sources have generally been considered unsuitable for industrial or medical applications.
One proposed solution was to keep the laser pulse repetition rate low to allow the crystal to cool and partially return to its initial state between pulses (as described in international application PCT/AU98/00554). However, in many industrial applications the low pulse repetition rate makes the application uneconomic due to slow material processing rates. Even in the medical applications of laser refractive surgery, the low pulse repetition rate can lead to impractically long treatment times. This is particularly true in the newer types of treatments based on topography or wave front linked customized ablations that require many smaller pulses to be applied to the cornea.
Thus, solid state UV lasers still have undesirable instability issues. With improvements in diode lasers in recent times there is now a preference that solid state lasers are diode laser pumped instead of flash-lamp pumped. Diode laser pumped solid state lasers are potentially more reliable and have better energy stability in their infra-red laser output than flash-lamp pumped solid state lasers. However, diode laser pumped systems are extremely inefficient at the low pulse repetition rates proposed in the solution mentioned above. Therefore, diode laser pumped solid state lasers, in particular, need a better solution to the instabilities of generating UV wavelengths through non-linear optical frequency conversions.